The present invention relates to a composition comprising a blend of at least three polyethylene components particularly useful for extrusion coating, a process using such blends for extrusion coating, and the composite produced thereby. The invention also relates to films produced from the blends.
Prior art polyolefin blends, containing linear low density polyethylene as a component, are useful in certain extrusion coating applications. One such prior art blend is Dow Chemical""s Dowlex 3010 linear low density polyethylene, which contains at least 80 weight percent linear low density polyethylene. Examples of extrusion coating applications are structures such as flexible polymeric film/paper packages for foods, and metallized polymeric film balloons. The linear low density polyethylene component in the prior art blends provides strong heat seals (coating-to-coating); strong tensile properties of coatings; strong resistance of coatings to tear; high resistance of coatings to missile, or dart, impact; resistance of coatings to puncture; and coating stress crack resistance. These properties are generally not attainable with polyolefin blends which contain no linear low density polyethylene. However, these prior art blends containing linear low density polyethylene exhibit certain undesirable processing problems.
The prior art blends, that contain high levels of linear low density polyethylene, i.e., 80 weight percent and greater, generally require excessive power (extruder motor drive watts) to be extruded during the extrusion coating operation. Frequently, the production rates of an extrusion coating operation must be slowed so as not to exceed the limits of the extruder drive motors. In many cases, productivity is so low as to result in economic losses.
Further, the prior art blends, that contain high levels, i.e., 80 weight percent and greater, of linear low density polyethylene, generally exhibit excessive extrudate edge bead, manifested by excessive neck-in of the molten extrudate. This is a problem readily known to those skilled in the art of extrusion coating. Excessive edge bead must be trimmed away and disposed of as scrap. Otherwise, a final coated structure will not exhibit a uniform thickness.
It would obviously be desirable in an extrusion coating operation to utilize a polyolefin composition containing linear low density polyethylene as a component which would extrude at significantly lower power requirements and with significantly less edge bead. It would further be desirable for that same polyolefin composition containing linear low density polyethylene as a component to retain the desired coating properties exhibited by the prior art two component polyolefin blends containing linear low density polyethylene as the major component of the two components. The properties include strong heat seals (coating-to-coating); strong tensile properties of coatings; strong resistance of coatings to tear; high resistance of coatings to missile, or dart, impact; coating puncture resistance; and resistance of coatings to stress cracking.
The present inventors have unexpectedly discovered an improved extrusion coating polyolefin composition comprising a blend of a linear low density polyethylene, a high density polyethylene, and a low density polyethylene. The composition of the present invention desirably extrudes with lower power requirements (extruder drive motor watts) than the prior art two component blends containing linear low density polyethylene. This occurs even though the compositions of the present invention require significantly more thermal energy per unit of mass to melt than is required by the prior art blends. The compositions of the present invention exhibit significantly less neck-in (less edge bead) than that exhibited by the prior art compositions.
The composition according to the present invention is a polyolefin blend comprising at least the following three components. The first component is a linear low density polyethylene which generally provides coating-to-coating heat seal strength, toughness, tear resistance, and puncture resistance. In general, the properties are essentially equal to those exhibited by the prior art compositions. The second component is a high density polyethylene copolymer which generally provides tensile strength and stress crack resistance properties. The properties are generally substantially equal to the prior art compositions. The third component is a low density polyethylene homopolymer which generally contributes to a reduced edge bead by providing a lower neck-in of the molten extrudate. Generally, this property of neck-in is superior to the prior art compositions. Moreover, the third component also generally provides good wetting of the substrate surface to which the blend is coated, for good coating-to-substrate bonding. Accordingly, the present invention that contains a lower amount of linear low density polyethylene than prior art compositions is utilized without a loss in coating properties that would be normally expected with a reduction in linear low density polyethylene content.
The present invention is also directed to an improved extrusion coating process comprising the use of the above-mentioned composition of at least a linear low density polyethylene, a high density polyethylene, and a low density polyethylene. Further, the present invention is also directed to a coated article or composite comprising a substrate, either primed or unprimed, having the aforementioned composition coated thereon, preferably by means of an extrusion coating process. The primer used to provide bonding to a substrate is preferably a water-soluble primer, more preferably a polyethyleneimine primer. Furthermore, the present invention includes laminates wherein a layer of the aforementioned three component composition is utilized between two similar or dissimilar substrates, optionally including a primer layer between the polyethylene blend and each substrate. The resultant composite or laminate may be formed into an article, such as a package for food, having good barrier properties, that is, the package is generally impermeable to liquids and gases. The present invention is also directed to films prepared from the novel compositions, particularly by means of cast or blown film techniques.
It is, therefore, an object of the invention to provide novel compositions comprising a blend of at least a linear low density polyethylene, a high density polyethylene, and a low density polyethylene.
Another object of the invention is to provide an improved extrusion coating process wherein there is utilized as the extrusion coating, the novel compositions comprising the blends of the present invention as defined herein.
It is a further object of this invention to provide a coated substrate or a composite wherein the coating composition comprises the novel blends of the present invention as defined herein.
A still further object of this invention is to provide a coated substrate or a composite wherein the coating composition comprising the novel blends of the present invention is applied onto the substrate by an extrusion coating process.
These and other objects and advantages of the invention will be apparent to those skilled in the art from the following detailed description, preferred embodiments, specific examples, and claims.